Conventionally, when a flexible circuit board is coupled to a liquid crystal display panel or when electronic components such as a semiconductor component, resistor, capacitor, or the like, are mounted on a circuit board, an anisotropic conductive adhesive or an anisotropic conductive sheet is used for electrically coupling terminals to each other.
FIG. 8A and FIG. 8B are sectional views showing an example of a conventional anisotropic conductive sheet and a coupling method thereof. FIG. 8A and FIG. 8B show a case in which an electronic component such as a semiconductor chip is coupled to a circuit board. However, the same method is employed in the case in which a semiconductor chip is directly coupled to a liquid crystal panel.
As shown in FIG. 8A, anisotropic conductive sheet 33 is obtained by dispersing conductive particles 35 in resin binder 34 that is a thermosetting adhesive resin and forming into a sheet. By using anisotropic conductive sheet 33, connection terminals 32 on circuit board 31 and electrode terminals 37 on electronic component 36 are coupled to each other. As shown in FIG. 8A, on circuit board 31, the above-mentioned anisotropic conductive sheet 33 is attached. Next, electrode terminals 37 on electronic component (for example, semiconductor chip) 36 are positioned to connection terminals 32.
Next, as shown in FIG. 8B, electronic component 36 is pressed against circuit board 31. By this pressing, the distance between electrode terminal 37 on electronic component 36 and connection terminal 32 on circuit board 31 is reduced, and electrode terminal 37 and connection terminal 32 are electrically coupled to each other with conductive particles 35. On the other hand, in a region other than a region between connection terminal 32 and electrode terminal 37, since anisotropic conductive sheet 33 is not compressed, the insulating property in the horizontal direction is secured. In this state, resin binder 34 is hardened. Note here that FIG. 8B shows that resin binder 34 becomes thermally hardened resin 38 after thermosetting.
In such a coupling method, in an anisotropic conductive sheet, low resistance is required in the vertical direction that is a coupling direction, and a high resistance state is required to be held and the adhesive strength is required to be improved between neighbors. In order to dissolve such problems, many developments have been done.
For example, Japanese Patent Unexamined Publication No. 11-306861 describes an anisotropic conductive film for preventing short circuiting between neighbors, and a coupling method using the same. This describes the use of an anisotropic conductive film including a mixture of a radiation curable resin that is hardened by radiation such as ultraviolet ray and a thermosetting resin, and conductive particles dispersed in the mixture. Furthermore, by attaching this anisotropic conductive film on a circuit board and, irradiation with radiation is carried out by using a mask having a light shielding portion in a position corresponding to a connection terminal on the circuit board. Next, the positions of electronic components are adjusted and pressurized, and then heated so as to be adhesively bonded. At this time, a region that has been previously irradiated with radiation is hardened and the movement of conductive particles in the horizontal direction is prevented. Therefore, short circuit between neighbors can be prevented.
Furthermore, Japanese Patent Unexamined Publication No. 2004-238443 describes an anisotropic conductive sheet using a resin in which hardening proceeds without need to carry out heat treatment by previously irradiating with light such as an ultraviolet ray so as to excite a hardening reaction as a method for simplifying steps of coupling electronic components on a circuit board. The coupling method using this anisotropic conductive sheet firstly excites a hardening reaction by irradiating the anisotropic conductive sheet with light, and then adhesively bonds electronic components to a circuit board while this anisotropic conductive sheet has tackiness, followed by holding at ordinary temperature. Thus, the hardening reaction is completed, and the electronic components are coupled to the circuit board.
The first example mentions that high density mounting can be achieved by preventing short circuiting between neighbors. However, when electronic components having different pitches or shapes of electrode terminals are mounted, a mask respectively corresponding to each of them is necessary. Furthermore, it is difficult to change the shape of the anisotropic conductive film in the region hardened by radiation. Therefore, in order to couple the electrode terminal on the electronic component and the connection terminal on the circuit board to each other via conductive particles with small coupling resistance, large pressing power is necessary. Recently, according to trend toward small and thin size, mounting of a semiconductor device with the thickness of 100 μm or less, a sheet-shaped device, or the like, has been required. However, such pressing power is applied to these devices, the devices may be damaged.
Furthermore, in the above-mentioned second example, by exciting a hardening reaction by previously irradiating with light, hardening can be carried out at ordinary temperature without need to carry out heat treatment. However, in this method, it is difficult to mount electronic components in which the intervals between electrode terminals and connection terminals vary.
Furthermore, in both examples, an anisotropic conductive film or sheet is attached only to a region on a circuit board in which an electronic component is mounted, followed by mounting electronic components on the anisotropic conductive film or sheet individually. In this method, the production steps become complicated.